Generally, a grain oriented electrical steel is used as iron cores for transformers and other electrical devices. For their magnetic characteristics, it is desirable to have a high magnetic induction and a low iron loss along the cold rolling direction. To have these characteristics, a grain oriented electrical steel has to be manufactured with cube-on-edge (110)[001] texture.
Such a (110)[001] texture is obtained through a secondary recrystallization, and this secondary recrystallization is a form of abnormal grain growth. That is, of the fine crystal grains produced through the normal recrystallization, grains of a particular orientation, i.e., grains with the (110)[001] orientation grow abnormally on the whole, thereby forming a secondary recrystallization. The driving force of secondary recrystallization is determined by the grain boundary energy and the size difference between the would-be secondary grain and fine primary grains. Therefore, if the growth of the secondary recrystallization grains of the orientation of (110)[001] is to be promoted, it is necessary to inhibit the growth of the primary recrystallization grains, and, in doing this, the method of adding precipitates such as MnS, AlN, BN and the like is used.
There have been proposed various techniques for manufacturing high magnetic flux density grain oriented electrical steel sheets, and one of these is U.S. Pat. No. 3,287,183. According to the method of this patent, silicon is added in the amount of 3%, and AlN and MnS are also added for inhibiting the growth of the primary recrystallization grains. Further, the final cold rolling reduction ratio is increased up to 81-95%, thereby increasing the magnetic flux density.
As a method of reducing the iron loss of the oriented high flux density electrical steel sheet, it has been proposed that the contents of silicon be increased, and the thickness of the sheet be decreased. However, if the silicon contents are increased, and the thickness of the sheet is decreased, then the secondary recrystallization becomes unstable, and the magnetic properties are deteriorated. Therefore, there is required a method of stabilizing the secondary recrystallization. As methods for stabilizing the secondary recrystallization under a high silicon content and a reduced sheet thickness, there have been proposed various techniques, and one of them is Japanese Patent Publication No. Sho-60-48886 which proposes to add Sn and Cu. According to this method, Sn is added in a range of 0.05-1.0% in order to stabilize the secondary recrystallization, and Cu is added in order to improve the deterioration of the glass film, which is caused by the addition of Sn.
The additions of Sn and Cu are effective for stabilizing the secondary recrystallization and for increasing the magnetic flux density. However, the additions of Sn and Cu markedly increases cracks on the surface of the hot rolled sheet coil, and these surface cracks cause fractures of sheets during cold rolling, thereby decreasing the productivity and yield.
Korean Patent Publication No. 91-2917 proposes a method in which 2 to 4 elements having low dissolvability to steel are added. That is, 2 to 4 elements selected from among Sn, Cu, Sb, Cr, Ni, Pb, Mo and Nb are added in amounts such that their ratio to the total weight of AlN+MnS should come within the range of 1-5. The patent asserts that, when such an addition is made, the growth of the fine precipitates of AlN and MnS is inhibited, thereby stabilizing the secondary recrystallization. However, in this method also, if the addition of Sn and Cu exceeds a certain level, cracks are formed during hot rolling, with the result that the actual yield is decreased.
Japanese Patent Laying Opening No. Sho-49-72118 presents a method of adding Cu in order to stabilize the secondary recrystallization. According to this method, Cu thus added forms Cu.sub.2 S by reacting with S existing within the steel, and this reinforces the inhibition of the grain growth in cooperation with the already existing inhibitors, thereby stabilizing the secondary recrystallization. However, according to the investigations of the present inventors, the addition of Cu gives no significant effect to stabilizing the secondary recrystallization, but rather gives adverse effects such as formation of surface cracks during hot rolling, and generation of decarburization defects.
Japanese Patent Publication Nos. Sho-57-14737 and Sho-56-4613 propose a method of adding Mo to oriented electrical steel sheet. The addition of Mo is done in order to prevent hot rolling cracks caused by S during hot rolling. This is considerably effective in preventing the surface cracks during hot rolling, but it can cause insufficient decarburization, if Mo is singularly added.